A liquid crystal projector, like other different types of electronic apparatuses, is equipped with a cooling fan for cooling their heat generating components. A filter is provided at the air inlet of these apparatuses to prevent grit and dust from entering the apparatuses.
Nevertheless, grit and dust accumulates in the filter, thereby eventually clogging the filter. Clogging of a filter will result in a pressure loss, so that clogging can be detected on the basis of a fan characteristic that the rpm of a fan increases with the loss in rotational load on the fan caused by pressure loss. For example, Japanese Patent Application Laid Open 2000-153121 teaches a filter clogging detection technique in which a voltage-rpm characteristic is defined for a cooling fan equipped with a clean filter and a determination is made that the filter is clogged as the deviation in current rpm from the voltage-rpm characteristic curve exceeds a given threshold value.
However, this prior art has a drawback in that filter clogging is not accurately detected when the projector is installed at a highland site, due to the fact that the atmospheric pressure changes with altitude and the characteristic curve is no longer adequate for accurate filter clogging detection in highlands.